A stratigraphic study of the Cretaceous Frontier Formation in the vicinity of Heart Mountain in the Big Horn Basin near Cody, Wyoming. and the Bridger Mountains on the western margin of the Crazy Mountains Basin in Montana has recently been completed. The formation is about 525 feet thick and consists of marine, deltaic, and lagoon sediments. It overlies the Mowry Shale Formation and is overlain by the Cody Shale Formation. A basal marine sandstone unit and a carbonaceous shale and mudstone facies have been mapped. The formation was deposited by progradation of sediment across south central Montana, southeastern Idaho, and northwestern Wyoming. Movement along major Precambrian fault systems may have displaced basement structures sufficiently to influence the progradation depositional process.